A couple months ago I went to London to research creepy Victorian settings for The Madman's Daughter series. I came back with an entire suitcase filled with resources, but this was my favorite find.
I found this London Street Map from 1891 at the Museum of London giftshop. My books take place in that same era, so it's perfect to know what was located where, which parks and streets and neighborhoods were different back then, and in general get a feel for how the city used to be laid out.
I put the map up on my office wall (it's huge) and then put colored sticky flags to mark specific places, such as where my characters might have lived, worked, shopped, and you know, commited murder. I also took some inspiring photos while I was there, such as this palace and townhome that make the perfect homes for some of my wealthier characters.
My husband calls it my "Dexter" board because all it needs is pieces of yarn connecting everything to be the kind of board half-crazy, obsessive detectives use to track serial killers. Not to mention how I tend to pace in front of it, muttering to myself about distances and shady alleyways. Did I mention my office doubles as our guest room? So yes, I get weird looks from guests who have to wake up to mapped-out images of Victorian crime scenes and creepy buildings.